Priority is claimed on Japanese Patent Application No. 2008-225817, filed Sep. 3, 2008, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for manufacturing an optical fiber preform by an OVD method, a VAD method, etc.
2. Description of Related Art
Methods, such as an OVD (Outside Vapor Deposition) method, a VAD (Vapor Phase Axial Deposition) method, an MCVD (Modified CVD) method, and a plasma method, are used for manufacturing an optical fiber preform.
Especially, the VAD method and the OVD method are well known as methods capable of realizing production of large size preform and high-speed deposition. In these methods, an optical fiber preform is obtained by forming a porous deposition portion, composed of glass fine particles, on a glass rod with tetrachlorosilane (SiCl4) or the like as a raw material to form a glass fine particle deposition and by transparentizing the deposition portion by heating.
The optical fiber preform is elongated to a predetermined diameter if necessary, and is obtained as a preform for an optical fiber, and this preform is heated and drawn and is obtained as an optical fiber.
In recent years, in order to reduce the manufacturing cost of an optical fiber, the diameter or length of the optical fiber preform is increased, that is, enlargement of the optical fiber preform is requested. However, if the optical fiber preform is enlarged, the following problems occur easily.
FIGS. 11 and 12 are views schematically showing an example of a heating furnace which heats a glass fine particle deposition.
A heating furnace 70 (hereinafter called “gradient furnace”) shown in FIG. 11 includes a muffle tube 71, a holding member 72 which is movable up and down, and a heating source 73 provided in a position equivalent to a portion of the glass fine particle deposition 4 in its length. The glass fine particle deposition 4 held by the holding member 72 is heated sequentially in the length direction by the heating source 73.
A heating furnace 80 (hereinafter called “soaking furnace”) shown in FIG. 12 includes a muffle tube 81, a holding member 82, and a heating source 83 capable of heating the glass fine particle deposition 4 over its total length.
The glass fine particle deposition 4 is brought into a state where an end of the glass rod 1 is held by the holding member 72 or 82, and the glass fine particle deposition is suspended into the heating furnace 70 or 80. The deposition portion 3 is heated to, for example, 1500 to 1600° C. by the heating source 73 or 83, and is transparentized.
In this transparentizing step, in order to utilize even the upper end of the deposition portion 3 as an effective portion without waste, it is necessary to sufficiently heat even this upper end.
However, when the upper end of the deposition portion 3 is heated, the glass rod 1 is also heated, and is softened. Thus, there is a possibility that deformation, such as elongation, may occur. Particularly, since the large-scale glass fine particle deposition 4 also has heavy weight, deformation of the glass rod 1 occurs easily.
In addition, the amount of heat required for transparentizing of the deposition portion 3 becomes larger as the diameter of the deposition portion 3 is large. Therefore, it is necessary to arrange the deposition portion 3 with large diameter in a high-temperature region within the heating furnace 70 or 80 or to lengthen heating time. For this reason, the amount of heat which the glass rod 1 receives also increases, and consequently, the above problem, such as deformation, also occurs easily. Since the glass rod 1 in which deformation has occurred cannot be reused, this result in a cost increases.
In order to prevent any deformation of the glass rod 1, the glass fine particle deposition 4 may be arranged in a position where the glass rod 1 does not become unduly high temperature, within the heating furnace 70 or 80. However, in this case, heating of the upper end of the deposition portion 3 may become insufficient, and this portion may not become an effective portion.
That is, with enlargement of the optical fiber preform, it was difficult to transparentize the deposition portion to its upper end, and to prevent any deformation of the glass rod.
The following method is known as the method for manufacturing an optical fiber preform.
In Japanese Unexamined Patent Application, First Publication No. 2003-81657, a method of adjusting the vitrification temperature, traveling speed, supply gas flow rate, etc. of an optical fiber preform according to the position of a vitrified portion of a porosity soot body, thereby preventing an optical fiber preform from falling from a supporting rod is suggested.
However, even in this method, it was difficult to make “transparentizing the porosity soot body to its upper end” and “prevention of any deformation of the supporting rod” compatible with each other as the optical fiber preform is made large. Additionally, there was also a problem in that it was difficult to cope with any variation in preform size.
In Japanese Unexamined Patent Application, First Publication No. H07-223833, a method of preventing a temperature rise in a rod which supports a soot deposition by a temperature rise preventing means, such as a reflecting plate or a heat shielding plate, is disclosed.
However, since a temperature rise preventing means is provided in this method, the structure of a heating furnace becomes complicated. Moreover, since an expensive temperature rise preventing means made of heat-resistant materials, such as porous ceramics, is required, cost may become high.
In Japanese Unexamined Patent Application, First Publication No. H08-310828, a manufacturing method using a preform holding portion including a central portion made of transparent quartz glass and an outer layer portion made of opaque quartz glass is suggested. In this method, the amount of transmission of infrared light was suppressed by the outer layer portion, and any deformation of the preform holding portion could be prevented. However, it was difficult to solve a problem of deformation of the rod.